Mozilla Firefox has decided not to add 64-bit support on their new Firefox version 4 (Note: Google Account required to read thread). While I don’t like it, there are still other options. Although, I’ve noticed quite a few sites out there that are programming for Firefox rather than Internet Explorer. Once Adobe implements a 64-bit version of Flash, I’m sure more browsers will be pushing the 64-bit envelope.
Early in the summer we decided to not yet provide supported 64-bit builds on Windows , and will instead work on delivering those in some future release.